1. Field of the Invention
The present invention is directed to a hearing aid of the type to be worn at the head, having a housing composed of two mating shells that accepts components of the device such as at least one signal receiver, an amplifier and transmission stage, an earphone and a battery.
2. Description of the Prior Art
German OS 11 39 549 discloses a hearing aid to be worn behind the ear that has a housing composed of two cover shells and a plate-shaped middle part in which a microphone, an amplifier, an earphone, a battery, etc. are arranged. In order to be able to utilize the full amplification of the device and in order to shield the microphone from the sound which emanates from the earphone and is conducted via the air ad via the hearing aid structure, the housing chamber which contains the earphone is sealed from the housing chamber which contains the microphone. To that end, sealing ribs projecting from the cover shells lie directly on the surfaces of the plate-shaped middle part and are intended to keep the acoustic waves of the earphone away from the microphone. Instead of ribs, sound-damping material can also be used, this being placed between the plate-shaped middle part and the cover shells. A combination of sealing ribs and sound-damping material is also possible. All of these measures are intended to serve for the suppression of feedback between the acoustic transducers.
Hearing aids with housing shells as in the case of hearing aids to be worn at the head and, in particular, to be worn behind the ear or to be worn at hearing aid glasses usually have thin plastic housing shells containing at least one microphone and/or an induction coil, an earphone, a battery as well as an amplifier and transmission part. In such hearing aids it has been found that oscillations of the housing parts and/or of the hearing aid components can occur due to self-resonance and/or outside excitation. In particular, a printed circuit board component of the amplifier and transmission stage can be effected by these oscillations. This leads to feedback problems which are different from the aforementioned feedback addressed in German AS 11 39 549. These different feedback problems associated with the amplifiers and transmission stage can cause malfunctions in the transmission behavior of the hearing aid.